The present invention relates to a green-emitting phosphor for a cathode-ray tube, in particular, of a projection video apparatus.
As shown in FIG. 1, a projection video apparatus comprises three projection cathode-ray tubes R, G and B which respectively emit red light, green light and blue light; lenses L for diverging the light rays from these tubes R, G and B, respectively; and a screen S for projecting the light rays emerging from these lenses, respectively.
In a projection video apparatus of this type, the phosphor screen of the cathode-ray tube is excited by an electron beam of a current density which is more than ten fold that of the conventional display cathode-ray tube. For this reason, the temperature of the phosphor screen is raised to 60.degree. C. or higher during the normal operation. The brightness of the phosphor screen is known to decrease with an increase in the temperature.
As a green-emitting phosphor used for a cathode-ray tube of a projection video apparatus is conventionally known which comprises zinc silicate (Zn.sub.2 SiO.sub.4 :Mn) activated by manganese of gadolinium oxysulfide (Gd.sub.2 O.sub.2 S:Tb) activated by terbium.
However, upon being excited with an electron beam, the Zn.sub.2 SiO.sub.4 :Mn phosphor has a luminous efficiency which is as low as about 7%, and tends to cause degradation on the phosphor screen which is known as a burn. Although the Gd.sub.2 O.sub.2 S:Tb phosphor has a luminous efficiency of 10% or higher, it has inferior temperature characteristics.